It is already known from the prior art the injection, by centrifugation, of the aluminum cages in rotors, which are formed by a stack of overlapped identical annular steel laminations provided with openings that are longitudinally aligned with the openings of other laminations of the stack, in order to define a plurality of axial channels interconnecting the external faces of the end laminations of the stack and which are angularly spaced from each other along a circular alignment, which is concentric to the longitudinal axis of the lamination stack, but radially spaced back in relation to the lateral face of the latter.
As illustrated in FIGS. 1–5 representative of the state of the art, the lamination stack 10, with the vertically disposed longitudinal axis, is positioned inside a mold 20, which defines a lower annular cavity 22 close to the external face of the lower end lamination, and an upper cavity 21, which is substantially cylindrical or frusto-conical, close to the external face of the upper end lamination and opened to the inlet channel 23 for the admission of aluminum into the mold 20.
During the pouring of the aluminum or of the selected metallic alloy, the lamination stack 10 has its central axial bore 11, into which will be later mounted the shaft of the electric motor, filled with a core 25, having an upper end substantially leveled with the upper end lamination of the lamination stack 10, and having a widened lower end portion seated on a respective lower end widening 11a of the central axial bore 11 of the lamination stack 10 and against the mold that defines the lower cavity 22.
The aluminum is poured into the upper cavity 21, passing through the axial channels 12 of the lamination stack 10 to the lower cavity 22, filling the latter, the axial channels 12, and the upper cavity 21, in this order, and solidifying in a radial inward upward pattern, as the mold 20 rotates around its vertical axis and the metal cools.
Upon completion of the aluminum pouring and solidification, the mold 20 is opened and the formed rotor is submitted to machining operations, in order to eliminate a projection 36 that has solidified in the inlet channel 23 (FIG. 4) and, subsequently, the aluminum plate that has molten against the upper end lamination of the lamination stack 10, in order to unobstruct the adjacent end of the central axial bore 11 of the lamination stack 10, and to define the correct internal profile for the upper ring 31 of the aluminum cage, which further comprises, in a single piece, a lower ring 32 already formed by the mold 20, and a plurality of bars 33 formed inside the axial channels 12 of the lamination stack 10.
In the centrifugation injection of these rotors, the upper cavity 21 and the lower cavity 22 of the mold 20 and the lamination stack 10 itself are heated, so that the molten aluminum passes through the upper cavity 21 and through the axial channels 12 of the lamination stack 10 without solidifying, gravitationally reaching the lower cavity 22, filling the latter and starting to solidify from the outside to the inside and from the bottom upwardly. Therefore, the upper cavity 21 and the lamination stack 10 are usually heated at a temperature much lower than the melting temperature of the aluminum, while the lower cavity 22 is heated at a lower temperature, allowing the aluminum to be solidified lastly at the hottest upper region of the mold 20. In order that the air existing in both the upper cavity 21 and the lower cavity 22, and in the axial channels 12 be conducted outwardly from the mold by the aluminum supplied to said mold, the lower cavity 22 is provided with an air outlet of any adequate construction (not illustrated).
As illustrated in FIGS. 1–5, the pouring of the aluminum into the mold 20 is effected through the inlet channel 23 and to the interior of the upper cavity 21, whose lower wall is defined by the upper end lamination of the lamination stack 10, and by the upper end of the rod 25 introduced in the central axial bore 11 of the lamination stack 10.
However, the liquid aluminum poured into the upper cavity 21 comes into direct contact with the internal marginal region of the upper end lamination of the lamination stack 10, which causes deformations in this region and allows the aluminum to penetrate between the upper laminations of the lamination stack 10.
Besides requiring extensive machining operations to form the upper ring 31, this prior art injection process requires a difficult machining of the central region of the upper ring 31 in order to adapt the oil pump of the compressor, due to the deformation of the upper end lamination in the uncovered central region of the upper ring 31. These machining operations reduce the productivity and increase the production cost of the rotors, particularly the rotors of small dimensions with a large volume of production. Furthermore, the penetration of aluminum between the laminations radially outwardly from the upper end region of the central axial bore 11 impairs the electromagnetic efficiency of the rotor.